1. Field of the Invention
This invention pertains to increasing the number of building blocks in oligonucleotides capable of independently forming non-standard Watson-Crick base pairs joined by patterns of hydrogen bonding different from those found in the adenine-thymine and cytosine-guanine base pairs.
This is a continuation-in-part of the patent application entitled "Non-Standard Base Pairs with Novel Hydrogen Bonding Patterns" Ser. No. 07/594,290, filed: Oct. 9, 1990, and issued Jul. 11, 1995 as U.S. Pat. No. 5,432,272. This continuation-in-part is copending with a continuation-in-part of the same application, entitled "In Vitro Selection with Non-standard Bases", Ser. No. 08/375,132, filed: Jan. 17, 1995. U.S. Pat. No. 5,432,272 discloses a method for synthesizing oligonucleotide analogs using DNA and RNA polymerases to incorporate nucleotides (non-standard nucleotides) capable of forming non-standard Watson-Crick base pairs joined by patterns of hydrogen bonding different from those found in the adenine-thymine and cytosine-guanine base pairs. The claims in U.S. Pat. No. 5,432,272 cover methods for synthesizing oligonucleotides containing non-standard bases. The claims in continuation-in-part Ser. No. 08/375,132 now pending cover compositions of matter that can be prepared by these methods. The claims in this continuation-in-part cover methods for using these compositions in a molecular recognition system.
2. Description of the Prior Art
Natural oligonucleotides bind to complementary oligonucleotides according to the well-known rules of base pairing first elaborated by Watson and Crick, where adenine (A) pairs with thymine (T) or uracil (U), and guanine (G) pairs with cytosine (C), with the complementary strands anti-parallel to one another. These pairing rules allow for the specific hybridization of an oligonucleotide with complementary oligonucleotides, making oligonucleotides valuable as probes in the laboratory, in diagnostic applications, as messages that can direct the synthesis of specific proteins, and in a wide range of other applications well known in the art. Further, the pairing is the basis by which enzymes are able to catalyze the synthesis of new oligonucleotides that are complementary to template nucleotides. In this synthesis, building blocks (normally the triphosphates of ribo or deoxyribo derivatives of A, T, U, C, or G) are directed by a template oligonucleotide to form a complementary oligonucleotide with the correct sequence. This process is the bases for replication of all forms of life, and also serves as the basis for all technologies for enzymatic synthesis and amplification of specific heterosequence nucleic acids by enzymes such as DNA and RNA polymerase, and in the polymerase chain reaction.
The Watson-Crick pairing rules can be understood chemically in terms of the arrangement of hydrogen bonding groups on the heterocyclic bases of the oligonucleotide, groups that can either be hydrogen bond donors or acceptors (FIG. 1). In the standard Watson-Crick geometry, a large purine base pairs with a small pyrimidine base; thus, the AT base pair is the same size as a GC base pair. This means that the rungs of the DNA ladder, formed from either AT or GC base pairs, all have the same length.
Further recognition between bases is determined by hydrogen bonds between the bases. Hydrogen bond donors are heteroatoms (nitrogen or oxygen in the natural bases) bearing a hydrogen; hydrogen bond acceptors are heteroatoms (nitrogen or oxygen in the natural bases) with a lone pair of electrons. In the geometry of the Watson-Crick base pair, a six membered ring (in natural oligonucleotides, a pyrimidine) is juxtaposed to a ring system composed of a fused six membered ring and a five membered ring (in natural oligonucleotides, a purine), with a middle hydrogen bond linking two ring atoms, and hydrogen bonds on either side joining functional groups appended to each of the rings, with donor groups paired with acceptor groups (FIG. 1).
Derivatized oligonucleotide building blocks, where a side chain has been appended to one of the nucleoside bases A, T, U, G, or C (the "normal" bases), have application because of their combination of Watson-Crick base pairing and special reactivity associated with the chemical properties of the side chain. For example, oligonucleotides containing a T to which is appended a side chain bearing a biotin residue can first bind to a complementary oligonucleotide, and the hybrid can then be isolated by virtue of the specific affinity of biotin to avidin (Langer, P. R.; Waldrop, A. A.; Ward, D. C. (1981) Proc. Nat. Acad. Sci. 78, 6633-6637), and finds application in diagnostic work. Oligonucleotides containing special functional groups (e.g., thiols or hydrazines) can be immobilized to solid supports more readily than those composed solely of the five "natural" bases.
Often, derivatized building blocks can be incorporated into oligonucleotides by enzymatic transcription of natural oligonucleotide templates in the presence of the triphosphate of the derivatized nucleoside, the substrate of the appropriate (DNA or RNA) polymerase. In this process, a natural nucleoside is placed in the template, and standard Watson-Crick base pairing is exploited to direct the incoming modified nucleoside opposite to it in the growing oligonucleotide chain.
However, the presently available base pairs are limited in that there are only two mutually exclusive hydrogen bonding patterns available in natural DNA. This means that should one wish to introduce a modified nucleoside based on one of the natural nucleosides into an oligonucleotide, it would be incorporated wherever the complementary natural nucleoside is found in the template. For many applications, this is undesirable. Many of the limitations that arise from the existence of only four natural nucleoside bases, joined in only two types of base pairs via only two types of hydrogen bonding schemes, could be overcome were additional bases available that could be incorporated into oligonucleotides, where the additional bases presented patterns of hydrogen bond donating and accepting groups in a pattern different from those presented by the natural bases, and therefore could form base pairs exclusively with additional complementary bases. The purpose of this invention is to describe compositions of matter containing these additional bases, and methods for using them to recognize complementary oligonucleotide strands also containing non-standard bases.